The present invention relates generally to CMOS integrated circuit techniques. More specifically, embodiments of the present invention provide methods and circuits for protecting amplifier output circuits.
Amplifier circuits are ubiquitous in modern electronic devices. An electronic amplifier increases the power and/or amplitude of a signal. In many applications, power amplifier circuits are used at the output stage of a system to drive an external device. Merely as an example, in an audio system, an output power amplifier is often used to drive an external speaker or headphone.
Power amplifier circuits output stages can be classified as class A, B, AB, and C, etc. for analog signal amplification. This classification is based on the portion of the input signal cycle during which the amplifying device conducts.
A Class A amplifier operates over the whole of the input cycle such that the output signal is a magnified replica of the whole input with no clipping. Class A amplifiers are the usual means of implementing small-signal amplifiers. In a Class A circuit, the amplifying device operated over the linear portion of its characteristic curve. Because the device is always conducting, even if there is no input at all, power is drawn from the power supply. Accordingly, class A amplifiers tend to be relatively inefficient, especially for large power devices.
In contrast, Class B amplifiers only amplify half of the input signal cycle. As such they tend to create signal distortion, but their efficiency is greatly improved over Class A amplifiers. This is because the amplifying element is switched off and does not dissipate power half of the time. An application using Class B amplifiers is the complementary pair or “push-pull” arrangement. Here, complementary devices are used to each amplify the opposite halves of the input signal. The amplified two halves are then recombined at the output. This arrangement gives improved efficiency, but can suffer from the drawback of mismatch at the “joints” between the two halves of the signal, also known as the crossover distortion. An improvement can be achieved by biasing the devices such that neither of the two devices is completely off when they're not in use. This mode of circuit operation is often called Class AB operation.
In Class AB operation, each device operates over half the wave similar to Class B operation, but each also conducts over a small signal range in the other half. As a result, when the waveforms from the two devices are combined, the crossover distortion is reduced. Here the two active elements conduct more than half of the time as a means to reduce the cross-over distortions of Class B amplifiers.
In certain applications, it may be desirable to use Class C amplifiers, which conduct less than 50% of the input signal and the distortion at the output is high, but high efficiencies are possible. An application for Class C amplifiers is in RF transmitters.
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to drive loudspeakers. Audio signals generally have frequencies between 20 Hertz to 20,000 Hertz, which is the human range of hearing. In a typical audio system, the audio amplifier is usually preceded by low power audio amplifiers which perform tasks like pre-amplification, equalization, tone control, mixing/effects, or audio sources like record players, CD players, and mp3 streams. Audio systems are used in public address systems, theatrical and concert sound reinforcement, and home sound systems, and mobile phones and tablets etc. The sound card in a personal computer often contains several audio amplifiers, as does every stereo or home-theatre system. Audio amplifiers often need to meet stringent performance requirement. In some applications, the input signal to an audio amplifier may measure only a few hundred microwatts. However, its output power may be tens or hundreds of watts.
Because of these requirements, Class AB push-pull circuits are a popular design choice in audio power amplifiers. Even though audio amplifier circuits are widely used in many applications, certain limitations still exist. Some examples are discussed below. FIG. 1A is a simplified view diagram illustrating an output portion 100 of a conventional audio system. As shown in FIG. 1A, an audio frequency signal 102 entering an amplifier 104, which amplifies the signal and drives a speaker 108. A schematic diagram of 100 is shown in FIG. 1B, where the amplifier is shown as a preamplifier 105 and a CMOS output driver circuit 106 including a PMOS driver device and an NMOS driver device. The speaker 108 is shown as an equivalent 8 ohm resistance load.
In some class AB amplifiers, a cascode output stage is used. A cascode amplifier usually has a common source amplifier as input stage driven by signal source. This input stage drives a common gate amplifier as output stage. The cascode configuration would offer a potentially greater gain and much greater bandwidth. It also enables the use of low voltage devices in the higher voltage circuit. This is the main reason to use a cascode in an output stage.
FIG. 2 is a circuit diagram of a conventional output circuit for a push-pull class AB cascode amplifier. As shown in FIG. 2, output circuit 200 includes first power node 201 for coupling to a positive power supply V0, second power node 202 for coupling to a negative power supply V1, and an output node 205. Output circuit 200 also includes first PMOS transistor P1 and second PMOS transistor P2 connected in series between positive power supply node 201 and output node 205. The drain node of P1 and the source node of P2 are connected at node 207. Output circuit 200 further includes first and second NMOS transistors N1 and N2 connected in series between output node 205 and negative power supply node 202. The drain node of N1 and the source node of N2 are connected at node 208. A first input node In1 is coupled to a gate of the first PMOS transistor P1. A second input node In2 is coupled to a gate of the first NMOS transistor N1. In FIG. 2, positive power supply V0 and negative power supply V1 are connected to a ground terminal GND. It can be seen in FIG. 2 that the gate of PMOS transistor P2 and the gate of NMOS transistor N2 are both biased at a ground voltage GND.